Porous crystalline materials like zeolites and organic/inorganic framework materials are highly desirable for gas adsorption and storage, catalysis, drug delivery, and sensing applications. However, one of the most daunting tasks in this field is to gain atomic-level control on the structure and composition of these materials. So far, trial and error in synthetic conditions has worked to achieve desired crystallinity and porosity. Moreover, small-scale synthesis can yield the desired structure, for the most part, by using specific solvents, temperature, and additives.

Despite these advances in synthetic control, bulk scale synthesis often fails because we lack understanding of nucleation mechanisms and the origin of periodicity. In a recent study, led by a group of scientists in India, the relationship between the structure of reactants and properties of the products was systematically studied and then used to make new materials.

Covalent Organic Frameworks (COFs) are a class of porous materials that are connected by covalent bonds between carbon and a “heteroatom” – nitrogen, boron, or oxygen. These materials contain long range periodicity in their structure, making them highly crystalline. In this work, the authors combined different combinations of trialdehydes and amine-acid salts to make the extended framework (Scheme 1).

Scheme 1. COF Synthesis Using a Series of Acid-Diamine Salts

Now, depending on the dissociation constant (Ka) and hence pKaof the acids chosen to make the amine-acid salts, the strength of their interaction with the amines will vary. If we think about the protonation of an amine as a function of strength of the acid, we can conclude that the stronger the acid, the more irreversible the protonation step will be (Scheme 2).

Scheme 2. Reaction Pathway to Make the COFs

The next step in the synthesis of the COFs is the imine (C=N) bond formation, which is also reversible. We have to keep in mind that to achieve high crystallinity, we want slower interaction between the reactants. If they react very fast, less long-range order is established, giving rise to poor crystals. Thus, reversibility in both the protonation and imine formation steps is desirable.

The crystal structures of these amine-acid salts show that each amine functionality (-NH3+) is surrounded by three acid molecules to form a chain-like one-dimensional layered structure (Figure 1). The researchers also calculated the hydrogen-bonding distances (dav), which in turn depend on the hydrogen-bonding strength. In Figure 1, six hydrogen bonding interactions are shown in dotted red lines between the amine hydrogen and the oxygen atom on the aldehydes (Namine‑H……Oacid).

Figure 1. Crystal structure of amine-acid salts.

Their study with ten previously characterized COFs showed higher crystallinity and porosity when the COFs are synthesized from phenol sulphonic acid compared to p-toluene sulphonic acid. More importantly, dav is 2.06-2.19Å when phenol sulphonic acid is used. So they hypothesized that to achieve highest porosity and crystallinity it is crucial for the reactant amine-acid salt to have dav in this ~2Å range. To prove the generality of their hypothesis further, the authors synthesized forty-nine new COFs and showed that COFs with this optimum range of dav values indeed have the greatest crystallinity and porosity. The synthesis of these new COFs suggests that the correlation between dav and the structure of COFs can be applied in general to synthesize wide variety of COFs with desired functionality.

This work reports a solvent-free solid-to-solid synthesis method for COFs, which is very promising from a greener and cleaner chemistry standpoint. The findings also shed light on the role of hydrogen bonding in the reactants on the porosity and crystallinity of the products. An atomic-level understanding of nucleation and crystallization in the synthesis of these porous materials is crucial for designing new materials. With better porosity we get higher gas adsorption in storage applications, and more crystallinity leads to better reactivity in catalysis and sensing applications.